The mechanical response of the peripheral auditory system in mammals has a significant nonlinear component, which is cochlear in origin and is observable in the external auditory meatus. Using automated methods of data acquisition aspects of the nonlinear response of the cochlea can be mapped in detail in both frequency and amplitude space. We have recently found that the observation of two tone intermodulation distortion products observed in the ear canals of cats using a certain protocol seem to map the nonlinearity in level space in its most fundamental way and show that nonlinearity can likely be described as a power law nonlinearity. The first experimental study proposed is to map thoroughly in level and in frequency several intermodulation distortion products to see if the preliminary indications of power law behavior of the nonlinearity are correct. To probe the nonlinearity from another aspect, we also propose to measure over a substantial range of frequency and level both the second and third harmonics to single pure tone inputs. Improvements in our experimental apparatus now allow us to make these measures over one hundred-fold a dynamic range in the level of these harmonics. We propose to supplement the experimental studies with a four stage theoretical study. The theoretical study, if successful, will allow us to summarize a large volume of data in a succinct mathematical picture. Since the nonlinear cochlear response is both a fundamental property of the peripheral auditory system and a sensitive indicator of both its structural and chemical integrity, its understanding is fundamental to an understanding of the cochlea. Moreover, since aspects of the nonlinear response can be measured noninvasively, its understanding and description can provide a useful diagnostic to cochlear health. This is a potential valuable control in animal experiments and has possibilities as a diagnostic in humans.